How can one prove by the structure of the fern that the fern never blooms?

To prove that ferns never bloom, you need to look under the leaf of this plant in late summer or early fall. Then there you can see small brown dots called sporangia. These containers contain spores (they are very similar to pollen), with the help of which ferns reproduce.
And flowering plants are not sporeal, they give fruits and seeds, such as apples, tomatoes, etc.